Recently, an adaptive optics (AO) technique for correcting even high-order wavefront aberrations by using an active optical element has been put into practical use and applied to various fields. In this technique, the wavefront aberration of probe light or signal light, which is generated by the characteristics of a measurement target itself or variations of the measurement environment, is sequentially measured by a wavefront sensor and corrected by a wavefront corrector such as a deformable mirror or spatial light modulator. At first, the adaptive optics (AO) was devised to improve the resolution by correcting the disturbance of the wavefront caused by fluctuations of the atmosphere at the time of astronomic observation. However, as an application field with a great effect of introduction, ophthalmic apparatuses which examine the retina of an eye are attracting attention.
As ophthalmic apparatuses, for example, there are known a fundus camera, and an SLO (Scanning Laser Ophthalmoscope) which acquires the two-dimensional image of a retina regarded as a plane. As another ophthalmic apparatus, an OCT (Optical Coherence Tomography) which noninvasively acquires the tomogram of a retina is also known and has already been in practical use. The SLO and OCT one- or two-dimensionally scan a light beam on the retina of an eye to be examined by a scanner, measure light beams reflected and backscattered by the retina in synchronism with each other, and acquire the two- or three-dimensional image of the retina.
The spatial resolution (to be referred to as a lateral resolution hereinafter) of the acquired image in the plane direction (lateral direction) of the retina is basically determined by the diameter of a beam spot scanned on the retina. To decrease the diameter of a beam spot focused on the retina, the diameter of a beam irradiating an eye to be examined is increased. However, the curved shapes and refractive indices of the cornea and crystalline lens, which is mainly in charge of refraction on the eyeball of an eye to be examined, are not uniform, and generate high-order aberrations on the wavefront of transmitted light. For this reason, even if a thick beam irradiates an eye to be examined, the spot on the retina cannot be converged to a desired diameter but spreads instead. As a result, the lateral resolution of an obtained image decreases, and the S/N ratio of an acquired image signal also decreases. Conventionally, therefore, a thin beam of about 1 mm, which is hardly influenced by the aberrations of the cornea and crystalline lens of an eye to be examined, is generally emitted to form a spot of about 20 μm on the retina.
As a means for solving this problem, the adaptive optics technique is being introduced. It has been reported so far that even if a thick beam of about 7 mm irradiates an eyeball by using this technique, the beam can be converged to about 3 μm, which is almost the diffraction limit, on the retina by wavefront correction, and a high-resolution SLO or OCT image can be acquired.
Japanese Patent No. 4157839 discloses an apparatus in which an adaptive optics system is applied to an SLO. In this adaptive optics system, a concave mirror for forming a collimated beam to irradiate a deformable mirror, and a concave mirror for receiving light reflected by the deformable mirror are adjacent to each other. This arrangement can minimize the entrance angle with respect to the concave mirror and thus can reduce the aberration of the optical system. Since ghost light reflected by the surface of an optical element forming the optical system can be suppressed, the accuracy of measurement of wavefront aberrations can be increased.
To minimize the entrance angle with respect to the concave mirror, the adaptive optical system needs to set a relatively long distance between optical elements, and the optical system becomes relatively large. The present invention provides a relatively compact adaptive optical apparatus, and an ophthalmic apparatus including the adaptive optical apparatus.